Atoms form molecules by means of their electrons. Electrons move very fast through the atoms in a way which makes it impossible to determine where an electron is located at a given time. There is only a statistical certainty that electrons are in a given zone around the nucleus of the atom. Which zone this is depends on the amount of energy that an atom has at a certain moment in time.

The different zones in an atom could be somewhat compared to an onion, whereby every layer could be a limited zone where electrons can be found. These layers are called “shells”. In the inner shell, closest to the nucleus of the atom there is only space available for 2 electrons. For H (hydrogen) with 1 electron and He (helium) with two electrons this is no problem. There is enough space available. But for Li (lithium) with 3 electrons another solution is necessary. So the third electron moves to the following shell which is situated more outwards from the nucleus. This 2nd shell can contain 8 electrons, just as the 3rd shell. The 4th and the 5th shell can contain up to 18 electrons, the 6th shell up to 32 electrons.

Lithium - Pumbaa

Photo: Lithium atom - author: Pumbaa

Every time a shell has been filled the other electrons are moving up to the next shell. Chlorine with 17 electrons has 2 electrons in the 1st shell, 8 in the 2nd shell and 7 in the 3rd shell. The more shells there are, the larger the atom. We can see this by comparing the diameters of C (6 electrons) and gold (79 electrons). The diameter of C is only about half the size of the diameter of gold.

Atoms continuously strive to reach full electron shells. They do this by taking up, giving off or sharing electrons with other atoms. It is a mechanism depending on chemical reactions and leading to the formation of molecules.

Covalent bonds

Let us look at the molecule “methane”. Methane consists of 1 carbon atom (C) and 4 hydrogen atoms (H). Carbon has 6 electrons, of which 2 are on the inner shell and 4 free electrons on its outer shell (2nd shell with ultimately 8 electrons). Carbon is 4 electrons short of a full outer shell. Hydrogen lacks one electron in its outer shell (1st shell with maximum 2 electrons). The carbon atom needs to combine with 4 hydrogen atoms to fill up its outer shell (4 + 4x1 electrons). Hydrogen only needs 1 electron to complete its outer shell. So the carbon atom unites with 4 hydrogen atoms, forming 4 electron pairs (4 x 2), resulting in full outer shells for all atoms. By doing this methane becomes a stable molecule.


Photo: Methane molecule - author not found

This kind of binding is called a “covalent bond”. The electrons are shared, on an equal basis, between the carbon atom and the 4 hydrogen atoms. That way each atom has, notwithstanding the motions of the electrons, always a full outer shell. The molecule has no extra free spaces anymore and therefore is in equilibrium. It has no need anymore to form new relations. It forms therefore a stable unit. Only when heavy collisions would occur or when enough external energy would be added, bonds could be broken.

The result is a chemical reaction. This is what happens when we burn methane as natural gas for cooking or heating. Methane molecules have no common interest in each other. They live in a loose connection that we call a “gaseous form”.

aardgas essent

Photo:Combustion of methane - author: Essent Energy

When we look at a water molecule we see a different story. Water exists of 1 oxygen atom with 8 electrons, (of which 2 are in the inner shell and 6 in the outer shell) and 2 hydrogen atoms (each with 1 electron on the outer shell). Oxygen needs 2 electrons to complete its outer shell. But oxygen is quite dominant and does not share the electrons on an equal basis. The oxygen atom sits between the two hydrogen atoms and in fact attracts the hydrogen electrons towards the center with as result that these electrons are closer to the oxygen nucleus than they would be in methane.

The result is that the hydrogen atom, because it partly looses a negative ion, receives a small positive charge. The opposite is true for oxygen. Because this atom attracts 2 negative electrons, it gets a more negative charge. We know that equal charges reject each other and that opposite charges attract each other. Because there are two slightly positive atoms in water these move slightly away from each other. You can see this in the drawing below. We call this effect “polarity” and we call this kind of molecule “polar”. Polarity has an effect on the properties of the molecule.

Dan Craggs-water  pl-olarity1.svg

Photo: Polarity of water - author: Dan Craggs

The reason why water molecules attract each other is due to the fact that the negative oxygen atom of one molecule attracts the positive hydrogen atoms of another water molecule. This way mutual connections are formed which we call “hydrogen bonds”. These bonds are partly responsible for the inner structure of cellulose and responsible for binding water in, for example, paper.


Photo: hydrogen bonds - author: Qwerter

Covalent bonds are mostly fairly weak bonds. Most molecules with covalent bonds have a liquid or gaseous form. The compounds they form mostly have relatively low melt- and cooking points.

Guy De Witte

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Atoms are also called “elements” because they are fundamental building blocks nature uses for creating life and for producing raw materials. Their properties, affinities, possibilities and limitations determine what kind of combinations they will enter into to form “molecules”. Other factors involved are the physical ambient conditions these atoms are embedded in, the presence of energy and/or matter that accelerates or promotes interactions.

Not only nature, but Man also, uses atoms to produce new molecules. The most innovative field of science involved in materials production is “nanoscience”. The practical exponent of this science is “nanotechnology”.

New materials and structures that are manufactured using nanotechnology, are produced by combining individual atoms into bigger structures. The size of these elements are less than 100 nanometer, hence the name nanotechnology (1 nanometer = 1 billionth of a meter = 10-9 meter or 0,000 000 001 millimeter). These new materials show other properties than those who are naturally occuring in nature or are used in traditional industrial manufacturing. That way nanotechnology opens a door to hundreds of new applications in the fields of mechanics, electronics, medicine, space exploration, airline construction, building materials and so on.


foto: nanotechnology in perspective  (author: Sureshbup)

Interactions between atoms

Atoms always want to interact with other atoms. Depending on the atom these interactions can occur with the same kind of atom, another kind or a combination of both. The result of these interactions are what we call “molecules”. What kind of interaction occurs depends again on internal and external factors.

So an oxygen atom (O) can react with another oxygen atom. The resulting molecule is O2 (oxygen gas) and hydrogen (H) can react with hydrogen to form H2 (hydrogen gas). But oxygen can also react with sulfur (S) to form SO2 (sulfur dioxide) or with carbon (C) to form CO2 (carbon dioxide) or with nitrogen (N) to form NO (nitrogen oxide). Hydrogen can react with oxygen to form water (H2O). The molecules formed that way can be gaseous like O2, liquid as H2O or solid as NaCl (natrium chloride= kitchen salt).


foto: carbon dioxide bubbles in gaseous water (author:Guy De Witte)

The chemical formula always shows how many atoms of each kind are involved in the molecule. So O2 consists of two oxygen atoms, H2 consists of two hydrogen atoms and H2O of two hydrogen atoms combined with one oxygen atom. Kitchen salt (NaCl) consists of one sodium atom combined with one chlorine atom. Glass mostly is composed of silicon dioxide (SiO2), formed by combination of one silicon atom and two oxygen atoms.


foto: silicon dioxide (author: LHcheM)

Some molecules are composed of multiple kinds of atoms. The pigment “white lead” for example has as formule Pb2CO3(OH)2 and contains two lead atoms, one carbon atom, five oxygen atoms and two hydrogen atoms. All abovementioned combinations are considered to be inorganic, as opposed to organic.

Organic compounds have molecules containing basically carbon (C) and hydrogen (H), often in combination with other atoms. They can form very complex structures. Simple organic compounds are CH4 (methane) and C2H5OH (ethylalcohol = ethanol). These simple compounds have mostly a ramified chain structure.


foto: representation of a methane molecule


foto: representation of an ethanol molecule (author: NEUROtiker)

Complex organic compounds show one or more ring structures in their molecules. Examples are C6H6 (benzene), C6H12O6 (glucose), building block of cellulose, and C8H8 (styrene) basic molecule for the production of polystyrene.


foto: glucose molecule (author: NEUROtiker)


foto: styrene molecule (author: Benjah-hmmm27)

to be continued


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To gain a better knowledge of the properties of objects and the dangers they are exposed to, it is important to have a better insight in their composition and the processes that have a negative impact on them.

This insight demands a basic knowledge of chemistry, physics and biology. Because not everybody involved in art and cultural heritage has this knowledge (maybe because it was never learned or maybe partly forgotten) we want to provide this basic knowledge through our blog/site. Sometimes we use a more simplified way for a better understanding.

Today we start with “THE ATOM”.

What is an atom?

The word “atom” is derived from the Greek word “atomos” which means ‘indivisible”. The ancient Greeks thought the atom was the smallest particle in the universe and that it was indevisible. Today this idea is outdated.

But the atom remains the smallest building block of which all matter and materials, as well vegetable as animal or mineral are composed of. Atoms are the “lego blocks” of nature. There are different kinds of atoms. They have a different composition and different properties and can be combined to form living materials or non-living materials. The way they combine happens according to coded instructions inherent to nature. These instructions can be of a physical or chemical nature. They are a kind of “manuals” according to which the building blocks are combined to form an entity. Because these “manuals” are often more “guidelines” instead of a strict program, it is possible to find a lot of variations on the theme, especially in living materials. Living creatures or objects composed from once living matter we call “organic”. Non-living materials we call “inorganic” (= non-organic).

How is an atom structured?

Atoms have a diameter of about one tenth of a million of one millimeter. This is very small and invisible for the naked eye even using a normal laboratory microscope.

An atom consists of smaller particles of which we will only mention the ones most important to our purpose: protons, neutrons and electrons. The most simple representation of an atom is comparable to our solar system: a central core (consisting of protons and neutrons) around which circulate smaller objects (electrons). In practice the system is more complicated. The electrons move at a crazy speed aorund the core and not in planetary orbit as our planets, but criss-cross through a spherical or elliptical space around the atom core.


Representation of atom (Adrien Facélina)

The amount of protons can be equal or not to the amount of neutrons. In an atom there are always as many protons as electrons. Protons have a positive electrical charge, electrons a negative electrical charge. This way the particles keep each other balanced. Neutrons do not have electrical charges.

The core of an atom is very small. The diameter is about 1/1000 of the total diameter of the atom. The electrons are smaller still. Everything in between is empty space, which allows for a lot of movement. All objects, even living creatures are thus composed of a lot of empty space.

There is a whole variation of atoms. This is based, amongst others, on the different amount of protons and neutrons in the core. The properties of these atoms are therefore different. Atoms can contain up to 92 protons. Based on the amount of protons the atoms have received what we call an “atom number”. All these atoms occur in nature. Number 92 is uranium. Every type of atom has specific properties and behaviour and react in specific ways with other atoms. There are also atoms with a higher atom number (up to 118 nowadays), but these atoms are not occurring spontaneously in nature. They can only be synthesized with scientific equipment.

The table of Mendelejev

Based on the diversity of properties the Russian Dmitri Mendelejev devised a clear table of elements for all known atoms in his time. Elements have been added to this table when they became available. This table shows the various relations between the different atoms.


Table of Mendelejev (DePiep)

Names and symbols

Each atom is represented by a letter symbol. Mostly, but not always these are the first letter(s) of the Greek or Latin name for the atom. So the letter H stands for Hydrogenium (Hydrogen), He for Helium (Helium) and Cl for Chloros (Chlorine).

In conservation practice we will encounter often the same atoms. That is why it is important to know the exact names, shortenings and properties of these atoms. Lacking the right knowledge can have a large impact on conservation and survival of materials.

The most occurring atoms we will work with are: H (hydrogen), C (carbon), N (nitrogen), O (oxygen), P (phosphor), S (sulfur), Cl (chlorine). Depending on the materials we work with we will also need knowledge of Na (sodium), Mg (magnesium), Al (aluminium), Si (silicium), K (potassium), Ca (calcium), Fe (iron), Ni (nickel), Cu (copper), Zn (zinc), Ag (silver), Sn (tin), Au (gold) and Pb (lead).

In a next post we will cover the interaction between identical and non-identical atoms.

Guy De Witte

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